1. Field of the Invention
The present invention relates to copper paste for forming a conductive thin film by baking, and more particularly, it relates to copper paste containing glass frit.
2. Description of the Background Art
Copper paste is employed as a conductive thick film for providing an electrode or an interconnection pattern on an electric insulating substrate or an electronic component body, for example. The copper paste is applied onto the insulating substrate or the electronic component body, and thereafter baked to form a conductive thick film.
The copper paste is generally obtained by mixing copper powder, glass frit and an organic vehicle. This copper paste first has an advantage of a low material cost. Further, a conductive thick film which is formed by the copper paste has excellent solderability, and hardly causes migration, which is easily caused in a conductive thick film formed by silver paste.
In general, the conductive thick film must have strong adhesive strength with respect to a surface to be formed, in addition to excellent solderability and small specific resistance. In order to simply improve the adhesive strength, the content of the glass frit is effectively increased. When the content of the glass frit is increased, however, the solderability is reduced and the specific resistance is increased. Further, such an increased content of the glass frit disadvantageously leads to reduction of the adhesive strength when the conductive thick film is exposed to heat hysteresis caused by soldering or the like. In Japanese Patent Application No. 62-326452 (1987) (Japanese Patent Application Laying-Open No. 1-167907 (1989), the assignee has proposed copper paste for forming a conductive thick film, which can solve these problems.
The copper paste proposed in the above application, which is obtained by mixing copper powder, glass frit and organic vehicle, contains not only spherical copper powder of 1 .mu.m to 10 .mu.m in particle diameter, but copper powder of less than 1 .mu.m in particle diameter. Due to such addition of the copper powder of less than 1 .mu.m in particle diameter, the thick film is densified after baking, to prevent penetration of solder and improved adhesive strength between the same and a surface to be provided with the thick film.
The inventors have noted the glass frit in order to attain further preferable properties while maintaining the aforementioned preferable characteristics of the copper paste, and improved the glass frit, to complete the present invention.
Copper paste which is of interest to the present invention is also disclosed in Japanese Patent Application Laying-Open Nos. 3-167713 (1991) and 4-277406 (1992), in addition to the aforementioned Japanese Patent Application Laying-Open No. 1-167907 (1989). However, none of these documents notes the particle diameter of the glass frit, although the same notes that of the copper powder. Generally employed glass frit is 4.0 .mu.m in mean particle diameter.